1. Technical Field
The present invention relates to titanium aluminide, i.e., an intermetallic compound known by a chemical formula of TiAl, as an advanced material for precision casting. It relates in particular to that species of titanium aluminide whose fluidity is excellent, the precision cast articles made therefrom have a high strength in cast state and will not crack even when their thickness is small.
2. Background Art
Titanium aluminide (an intermetallic compound known by a chemical formula of TiAl referred to as "TiAl" hereinafter) is drawing attention as an advanced material for its higher specific strength at high temperature than those of the nickle-base superalloys and better oxidation resistance than those of the titanium alloys. Since TiAl has other admirable properties in addition such as low density, the strength of which becomes greater with elevating temperature and good creep resistance, there are demands to make aircraft jet engine parts such as blades and vanes out of this material in the form of thin and intricately configured precision cast articles.
On the other hand, however, TiAl is known to have low ductility at ambient temperature and have a strong dependency on the deforming speed even at high temperatures where sufficient toughness develops. To overcome these difficulties, research is being conducted from crystal structural and physical metallurgical viewpoints. For example, methods of improving the low ductility by strengthening the grain boundaries have been proposed in Japanese Patent Application Nos. 41740/1986, 255632/1989, 2874243/1989 and 298147/1989 and in U.S. Pat. No. 4,294,615.
Despite these efforts, however, the reality is that precision cast articles made of binary Ti--Al alloy remain so liable to cracking that they cannot be called an industrial product. Even with addition of a third element, e.g., V, which the above-mentioned U.S. Patent has found effective to improve a ductility, ternary Ti--Al--V alloys containing appreciable amounts of the third element, e.g., V as much as 1.5 mass %, cannot make castings, such as turbine vanes, perfectly crack-free.
Furthermore, even while the above-cited Japanese patent applications claim to produce TiAl cast articles having strengths surpassing those mentioned in the specification of U.S. Pat. No. 4,294,615, the strengths achieved at ambient temperature are in the 400 MPa level; even with addition of a strength improving element as in JPA No. 255632/1989, strengths over 500 MPa have not been realized.
For another thing, there is an observation that the poor toughness of TiAl should be considered as due, on top of the inherent brittleness of this material arising from its being an intermetallic compound, to the coarse lamellar grains that characterize its microstructure. Here, it is to be noted that the stoichiometric titanium aluminide, i.e., the one that corresponds to an Al content of 36 mass %, does not develop the lameller structure, but this material has a lower ductility than a lameller structured TiAl. With these so-called industrial TiAl alloys, which are generally of an Al content of 32 to 34 mass % because of the addition of property-modifying elements of one sort or another, on the other hand, development of the lameller structure has been considered inevitable.
As a countermeasure thereto, a proposal has been made to add B or Y so as to strengthen the lamellar grain boundaries. Even then, however, attainment of acceptably low rates of rejection is often impossible when the product is a thin and intricately configured cast article such as turbine blades because these coarse lamellar grains still induce cracking.
Now, those thin and intricately configured articles such as turbine blades and impellers are commonly manufactured by the precision casting (e.g., the lost wax or investment casting) method because other methods such as precision forging and machining are generally very difficult. Here, to ensure good fluidity (i.e., the ability of the molten matter to fill up the casting mold or cavity to its tips) for the material is a must to attain a high yield of good castings or low enough rejection rates. In the case of TiAl, however, deterioration of the rejection rate is simply inevitable if an additive such as Mo, V and Nb has been added in a large quantity even for the sake of improving the toughness, because such an addition inevitably raises the melting point, enlarges the solidification temperature range and decreases the melting latent heat, all contributing to aggrevate the fluidity. In particular, the melting temperature having been elevated means that Ti is activated that much and its reaction with the casting mold is promoted that much, thereby making sound casting that much more difficult.